1. Field of the Invention
The present invention relates to a converting apparatus provided with a converter including a plurality of converter units connected in cascade.
2. Description of the Prior Art
A converter for converting an AC current into a DC current basically comprises one set of full-wave rectification bridge circuits, among which there is a converter including a plurality of bridge circuits connected in cascade, thereby enabling output control within a wider range. FIG. 1 is a schematic circuit diagram of a converter disclosed in "Thyristor Phase-Controlled Converters and Cycloconverters" by B. P. Pelly, published by Wiley Interscience in 1971. This converter includes two six-phase bridge circuits, called the converter units 1 and 2 in this specification, which are connected in cascade with each other. This connected one is called the converter in this specification and the entire converter inclusive of a commutation control circuit (not shown in FIG. 1) is called the converter apparatus therein.
A three-phase AC power source 8 is connected to the primary winding 51 of a step-down insulating transformer 5, the secondary winding 52 and the tertiary winding 53 thereof which are equal in the number of windings, being connected to AC input terminals at the converter units 1 and 2 respectively, the primary and secondary windings being delta-connected and the tertiary winding being star-connected, whereby AC voltage applied to the converter units 1 and 2 has a phase difference of 30.degree..
The converter units 1 and 2 consist of silicon controlled rectifiers SCR 4 connected in a six-phase bridge manner so that DC outputs of the converter units connected in cascade are given to a load 3. In FIG. 1, reference e designates voltage of AC power source 8, e.sub.oa and e.sub.ob designate the secondary and tertiary voltages of the transformer 5 respectively, Voa and Vob designate each output voltage of the converter units 1 and 2, Vo designates the sum of Voa and Vob, that is, output voltage of the converter, and Io designates a load current.
For commutation control for the converter, there are two systems, that is, an asymmetric control system which always keeps an output of the one converter unit maximum and adjusts an output of the other, and a symmetric control system which simultaneously adjusts both the converter units to be equalized to each other.
FIG. 2 is a vector diagram at the AC base in the asymmetric control system when voltage Vo of the converter output is 1, 0.75 or 0.25 (corresponding to (i), (ii) or (iii) in FIG. 2 respectively), in which, when both the converter units 1 and 2 output the maximum voltages, Vo corresponds to the above mentioned 1, and the output Voa of converter unit 1 is kept constant to thereby adjust the output of the converter unit 2. Each reference .phi.o designates a phase angle between Vo and Io.
FIG. 3 is a vector diagram showing the AC bases in the symmetric control system when the converter output voltage Vo is 1, 0.75 or 0.25 as the same as FIG. 2.
FIG. 4 is a circle diagram showing a relation between the output voltage Vo of the converter and the reactive power of a fundamental wave at the AC power source 8, which is represented by a ratio in the case when the asymmetric control is applied with respect to the case when the symmetric control is applied, with keeping Io constant. In the case of symmetric control, when the output voltage is Vo.sub.3 (Voa=Vob=Vo.sub.3/2), the reactive power Q.sub.1 is generated. On the contrary, in the case of the asymmetric control (Vo.sub.3 =Voa+Vob and Voa&gt;Vob), the reactive power is Q.sub.2, whereby the reactive power may be smaller by Q.sub.3 (=Q.sub.1 -Q.sub.2) than that in the case of the symmetric control. In FIG. 4, the SCR 4 constituting the converter units 1 and 2 is assumed to operate as an ideal switch.
As above mentioned, the asymmetric control system is advantageous in that the reactive power is less to create, however, since a phase control angle of SCR 4 is fixed to allow one converter unit to always output the maximum voltage, a cycle period adjustable of the output voltage Vo becomes two times as long (a cycle period of 60.degree. in the example of FIG. 1) as that of the symmetric control system. Accordingly, the asymmetric control system is inferior to the symmetric control system in that precision control is impossible when Io is adjusted by adjusting Vo. When both converters simultaneously carry out the commutation, the asymmetric control system is inferior to the symmetric control system in that the ripple in a load current Io is larger than that of the latter, because the output of the one converter unit in the asymmetric control system is kept maximum and the ripple of the output voltage is larger than that of the symmetrical control system.